Examine source code of Custom 404 Page Notifier

{
  "manifest_version": 2,
  "name": "Custom 404 Page Notifier",
  "version": "1.0",
  "description": "Notifies users when they encounter a 404 page.",
  "permissions": [
    "tabs",
    "webNavigation"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "custom_404_page_notifier_extension@gmail.com"
    }
  }
}